Recent large randomized controlled trials (RCTs) for cLBP indicate that acupuncture is superior to both optimal guideline-based mainstream care and waitlist controls. Yet, the medical community's acceptance of acupuncture as a treatment option has been hampered by a failure to find significant clinical differences between verum and sham (placebo) acupuncture. Importantly, despite their similar clinical effect, studies from both our and other groups found that real and sham acupuncture treatments work through different brain mechanisms to produce treatment effect. In parallel, previous studies have demonstrated that context (i.e., the broad non-specific effects of treatment ncluding patient-practitioner relationship, expectation, warmth, support, attention, and elaborate ritual) can have significant and meaningful impact on many disorders. Recent RCT trials by our group using acupuncture as the method of treatment also demonstrated that, relative to treatment delivered with a limited provider engagement, treatment delivered with augmented context effect produces a significant increase in clinical benefit. The relative ease by which acupuncture efficacy can be systematically varied by context makes acupuncture an ideal choice for future context experiments. Notably, neural longitudinal mechanisms underlying the two context conditions (limited versus augmented) have yet to be studied. We propose to conduct a novel mechanistic longitudinal 2x2 factorial study on cLBP patients using the cutting edge neuroimaging technique of arterial spin labeling (ASL) and BOLD fMRI. Eighty patients with cLBP will be randomized to one of five arms: A) "augmented context" placebo (sham acupuncture);B) "limited context" placebo (sham acupuncture);C) "augmented context" verum acupuncture D) limited context" verum acupuncture. The endpoints will include both clinical cLBP outcomes and neuroimaging biomarkers. We believe that this study will elucidate the mechanisms underlying both context effect and acupuncture, potentially translating to maximization of clinical treatment benefits for cLBP and/or use of alternative treatments.